1. Field of the Invention
The present invention relates to a display device in which the viewing angle can be switched, a terminal device in which the display device is mounted, and a display panel incorporated in the display device.
2. Description of the Related Art
In recent years, because of their thin profile, light weight, small size, low power consumption, and other advantages, display devices that use liquid crystal have been widely mounted and used in monitors, televisions (TV), and other large terminal devices; notebook computers, cash dispensers, automatic vending machines, and other midsized terminal devices; and personal TVs, PDAs (Personal Digital Assistants, mobile phones, portable game machines, and other small terminal devices. These liquid crystal display (LCD) devices can be classified into transmissive-types, reflective-types, and semitransmissive-types in which transmitted and reflected light are used in combination, in accordance with the type of light source that is used. A reflective-type device can reduce energy consumption because the device can make use of external light for display purposes, but since the contrast and other display properties are inferior in comparison with transmissive-type devices, transmissive-type and semitransmissive-type devices are currently the mainstream devices. In transmissive and semitransmissive LCD devices, a light source device is disposed on the back surface of the liquid crystal panel, and the display processes are implemented using the light emitted by the light source. Specifically, a light source device is currently required in addition to the liquid crystal panel in a mainstream LCD device.
A liquid crystal panel, which is a main constituent element of a LCD device, uses an electric field to control the orientation of liquid crystal molecules and to display information. Many modes have been proposed by combining the type of liquid crystal molecules, the initial orientation, the direction of the electric field, and other features of the device. The most often used among these modes in conventional terminal device are the STN (Super Twisted Nematic) mode in a passive matrix structure, and the TN (Twisted Nematic) mode in an active matrix structure, but liquid crystal panels that use these modes have a narrow range of angles in which a grayscale to be correctly viewed, and the grayscale is inverted when the viewer is not situated in the optimal viewing position.
The issue of an inverted grayscale was not a major problem in mobile phones and other terminal devices when the display content was telephone numbers and other character-based content. Due to recent advances in technology, however, terminal devices now display a large amount of character information as well as image information. Therefore, image viewability is dramatically reduced due to an inverted grayscale. For this reason, liquid crystal panels having a wide viewing angle mode that allows a grayscale to be correctly viewed without a grayscale inversion are gradually being mounted in greater numbers in terminal devices. Liquid crystal panels having such a mode are generally referred to as liquid crystal panels having a wide viewing angle, and these panels employ multi-domain vertical orientation modes and horizontal electrical field modes such as IPS (In-Plane Switching). Since a correct grayscale can be viewed in a wide range of viewing angles by using these liquid crystal panels having a wide viewing angle, applications are being developed and panels are increasingly being mounted in such applications in order to share information with others and simultaneously view images with more than one person, even though small and midsized terminal devices are essentially personal tools.
On the other hand, small and midsized terminal devices are also used in public places as well as in rooms having strict security. In such cases, security is required that does not allow private and confidential information to be viewed by a third party. Recently in particular, the opportunities for displaying private and confidential information have increased together with the advancement in terminal devices, and the need for a technique that prevents unwanted viewing has increased. In view of this situation, there is a need for a technique that allows viewing by only the user, but prevents unwanted viewing by narrowing the viewable angle range, i.e., the range of viewing angles of the display.
As described above, there is a need for a display that has a wide range of viewing angles to allow simultaneous viewing by more than one person, and that has a narrow range of viewing angles to allow viewing only by the user. There is also a need for a terminal device that can switch between these two types of displays. To satisfy such a need, display devices have been proposed in which the light source device, which is an essential part of a LCD device, has been modified to allow the range of viewing angles to be varied.
FIG. 1 is a cross-sectional diagram schematically showing a first conventional viewing-angle-controllable LCD device described in Japanese Laid-open Patent Application No. 5-72529 (hereinafter referred as “Patent Document 1”). As shown in FIG. 1, the first conventional viewing-angle-controllable LCD device 1001 comprises a liquid crystal element 1170 that can control scattering, and a liquid crystal element 1180 that can control optical rotation and birefringence. The liquid crystal element 1170 that can control scattering has substrates 1110 and 1111 that are optically transparent in the visible range, transparent electrodes 1120 and 1121, a scattering liquid crystal 1130, a power supply 1100, and a switch 1190. The liquid crystal element 1180 which can control optical rotation and birefringence has substrates 1111 and 1112 that are optically transparent in the visible range, transparent electrodes 1122 and 1123, polarizers 1140 and 1141, orientation films 1150 and 1151, a liquid crystal layer 1160 that has optical-rotation and birefringent characteristics, a power supply 1101, and a switch 1191. A polymer dispersion liquid crystal is used as the scattering liquid crystal 1130, and TN liquid crystals are used as the liquid crystal element 1180 that can control the optical rotation and birefringence. The polarizers 1140 and 1141 are disposed in a crossed-Nicol configuration.
In the first conventional viewing-angle-controllable LCD device configured in the manner described in Patent Document 1, the optical rotation and birefringence of the liquid crystal layer 1160 is varied by applying a voltage between the transparent electrodes 1122 and 1123, and this variation can be used to control the transmissivity of the light. In a display mode that uses such optical-rotation and birefringent characteristics, a phenomenon occurs in which the brightness and hue are reduced or inverted depending on the viewing angle, because the optical rotation and birefringence applied to the incident light are substantially different depending on the viewing angle.
In view of the above situation, the liquid crystal element 1170 that can control scattering is disposed above the liquid crystal element 1180, which has such viewing-angle dependency, and the viewing angle dependency is decreased. In other words, the liquid crystal molecules are arranged at random when an electric field is not applied to the liquid crystal 1130 of the liquid crystal element 1170 that can control scattering. Therefore, light is substantially isotropically scattered across the entire viewing angle, and a display with a low viewing-angle dependency can be obtained. On the other hand, when the electric field is applied to the liquid crystal 1130, the light emitted from the liquid crystal element 1180 is emitted unchanged without being scattered by the liquid crystal molecules because the liquid crystal molecules are aligned substantially parallel to the applied electric field. In this case, the viewing angle characteristics are not improved and are similar to the viewing angle characteristics of a conventional TN liquid crystal. However, only the user positioned in front of the screen can correctly view the image. Therefore, when only one user located in front of the screen needs to be able to correctly view the screen, the electric field is not applied to the liquid crystal 1130, whereby unwanted viewing by others can be prevented.
FIG. 2 is a cross-sectional diagram schematically showing a second conventional viewing-angle-controllable LCD device described in Japanese Laid-open Patent Application No. 9-244018 (hereinafter referred to as “Patent Document 2”), and FIG. 3 is a perspective view schematically showing the illumination device used in a conventional viewing-angle-controllable LCD device. The second conventional viewing-angle-controllable LCD device 2101 comprises a LCD element 2102, a scattering control element (scattering control means) 2103, and an illumination device (backlight) 2104, as shown in FIG. 2. The scattering control element 2103 is disposed between the LCD element 2102 and the illumination device 2104. The illumination device 2104 is disposed below the scattering control element 2103, and comprises a sheet (translucent sheet body) 2120 having light-blocking slits, and an irradiation portion 2121, as shown in FIG. 3. Fluorescent tubes or other light sources 2122 are disposed in the irradiation portion 2121, and a light-excident surface 2123 is formed for emitting light from the light sources 2122 and directing the light to the sheet 2120 having light-blocking slits. A reflection sheet 2124 for reflecting the light emitted from the light source 2122 is disposed on the surface facing the light-excident surface 2123 in irradiation portion 2121. The sheet 2120 having light-blocking slits has a large number of linear light-blocking materials that are disposed parallel to each other on one surface of a transparent sheet. The direction in which the light-blocking material extends is aligned with the vertical direction of the display screen.
In the second conventional viewing-angle-controllable LCD device configured in the manner described in Patent Document 2, the light emitted from the light source 2122 is emitted from the light-excident surface 2123 of the irradiation portion 2121 and is irradiated toward the scattering-control element 2103 by way of the sheet 2120 having light-blocking slits. When the light emitted from the light-excident surface 2123 passes through the sheet 2120 having light-blocking slits, the sheet 2120 having light-blocking slits blocks the light that enters from the direction that is considerably inclined toward the light-incident surface of the sheet 2120 having light-blocking slits. Transmitted light can thereby be obtained that is highly parallel in the vertical direction with the surface of the sheet 2120 having light-blocking slits. The light emitted from the illumination device 2104 enters the scattering-control element 2103. The scattering-control element 2103 controls the scattering of light beams that have entered in accordance with the presence of applied voltage. When the scattering-control element 2103 is in a scattering state, the light emitted from the illumination device 2104 is scattered by the scattering-control element 2103, and when the scattering-control element 2103 is in a transparent state, the light emitted from the illumination device 2104 is not scattered.
In the second conventional viewing-angle-controllable LCD device 2101, when the scattering-control element 2103 is in a scattering state, highly parallel light emitted from the illumination device 2104 is scattered by the scattering-control element 2103 and received by the LCD element 2102. As a result, light that has passed through the LCD element 2102 passes through all viewing angle directions of the display unit, and the display content can be viewed from positions other than the position directly in front of the display unit. Conversely, when the scattering-control element 2103 is in a transparent state, highly parallel light emitted from the illumination device 2104 is not scattered by the scattering-control element 2103, and highly parallel light enters the liquid display element 2102 unchanged. As a result, light is not transmitted, the display unit is dark, and the display content cannot be viewed from a position at a diagonal from the right and left in the horizontal direction. In other words, only the user positioned directly in front of the display unit can view the display content.
As described above, the second conventional viewing-angle-controllable LCD device 2101 can use the scattering-control element 2103 to control the scattering of light, and can therefore control the viewing angle characteristics of the display content. Also, since highly parallel light can be emitted by the illumination device 2104 toward the LCD element 2102, viewing angle characteristics can be reliably obtained in which the display content can be viewed only by the user positioned directly in front of the display screen when the scattering-control element 2103 is set to a transparent state. Therefore, the display characteristics have low viewing angle dependency, and a LCD device can be obtained that can arbitrarily switch between a state in which the display content can be viewed from only directly in front of the display, and a state in which the display characteristics are uniformly maintained across all viewing angle directions.
Nevertheless, the conventional viewing-angle-controllable LCD devices described above have the following problems. In the conventional viewing-angle-controllable LCD devices described above, another drive power source is required to drive the scattering-controlling liquid crystal element, and it is difficult to reduce size and costs. In particular, the scattering-controlling liquid crystal element must be driven using an AC power source because burn-in occurs when a DC power source is used. Accordingly, the drive power source becomes complex, and it is difficult to reduce the size and costs.